One candidate electric motor type for driving wheels of electric and hybrid vehicles is the induction motor. Induction motors, of course, may be designed in many different sizes and shapes for delivering rotational power.
A typical induction motor has a stationary annular wire-wound outer member of designed diameter and length called a stator. Often a three-phase alternating current is delivered to electrical leads of the stator so as to produce a magnetic field that rotates around the stator ring. A cylindrical rotor member carried on the rotating power shaft for the motor is placed closely spaced within the inner cylindrical cavity of the stator. The rotor has an inner cylindrical core of flat round steel plates, coated with electrically insulating material, and stacked as laminations with their circumferences aligned to form the cylindrical core so that it has a length complementary to that of the stator. This cylindrical core does not conduct electricity but it displays high electromagnetic permittivity.
Each laminated disk of the rotor core may be shaped with circumferential indentations, or the like, to carry several (e.g., 20-40) uniformly spaced, equal length, copper or aluminum electrical conductor bars extending from one end of the rotor core to the other. The spaced conductor bars may be uniformly slightly inclined to the cylindrical axis of the rotor core and the ends of each bar are connected to copper or aluminum end rings located on the rotor ends and co-axial with the rotor axis. This one-piece, cage-like structure of spaced and inclined conductor bars with end rings, carried on the laminated rotor core, is highly electrically conductive and termed a “shorted structure.”
Because only a small clearance is maintained between stator and rotor, the rotating magnetic field of the stator enters the rotor, inducing a current in the embedded conductors. In turn, the conductor current produces its own magnetic field which is repelled by the stator magnetic field and causes the rotor to rotate. Inclination of the conductor bars with respect to the rotational axis of the rotor cooperates with the rotation of the magnetic field produced by the stator and permits a more uniform production of torque by the induction motor.
The shorted structure may be fabricated by assembly and joining of its individual components, the conductor bars and end rings. An alternative approach, which promised a shorter manufacturing time, has been to overcast the conductor bars and end rings as a complete structure on the lamination stack using die casting. However, rotors manufactured using the die casting approach have exhibited problems with excessive porosity and lower than optimum shorted structure (electrical) conductivity which has reduced process yield.
Thus there is need for a process for rapidly fabricating induction motor rotors and particularly the shorted structure of such rotors.